Heurodis

Heurodis is a medusawizard who was once an apprentice to the dread lich Belpheron, a scholar of ancient Netheril. She somehow managed to escape undetected when the Harpers destroyed Belpheron and his other apprentices. Years later she began a plot to claim the ancient Netherese magic for herself and subject all Faerûn to herself. This set in motion the events of the Shadows of Undrentide campaign, where she is featured as the final boss.

The key to her plot was the acquisition of a mythallar, a crystalline Netherese power source. Her first attempt to acquire this involved hiring J'Nah to steal one that was possessed (unknowingly) by Drogan Droganson. At this point in the adventure, Heurodis is known simply as the hooded figure seen talking to J'Nah.
After J'Nah's failure, the crystal was entrusted to the hero of the campaign for delivery to an expert in the field of dweomer analysis. It was hoped that this expert, Garrick Halassar, would be able to ascertain the nature of the crystal. Garrick was at the time in the Anauroch, coincidentally studying the very Netherese ruins where Heurodis was preparing. Upon sensing the mythallar's approach, Heurodis made a public show of massacring nearby archeologists in the hopes that the one holding the mythallar would come to investigate. As she had assumed that one would be Drogan, her preparations were more than adequate to accommodate his newly-graduated apprentice. She petrified the hero of the campaign, claimed the mythallar for herself, and departed into the formerly-flying city of Undrentide.

With the power of the mythallar, Heurodis was able to bring the ancient city to life and caused it to once again fly. In the process, she was transformed, losing her petrifying medusa stare but gaining the power of the undead. Atop the Temple of the Winds and surrounded by the mythal—large crystals similar to the mythallar—no harm could come to her as she prepared to conquer the world.
However, the hero of the campaign recognized the weakness of her position and destroyed the mythal. While this removed the city's ability to fly, it also removed Heurodis' invulnerability, allowing her to be defeated (before the city crashed back to the ground).